1. Field of the Invention
The present invention relates to an f.theta. lens used in a scanning optical system, and more particularly to an f.theta. lens for forming a fine spot and a laser scanning optical system using the same.
2. Related Background Art
In a prior art laser scanning optical system, a laser beam emitted by a laser light source is collimated by a collimater lens, reflected by a deflector such as a polygon mirror, and a laser beam spot is formed on an image plane by a focusing lens system and it is scanned.
In such a laser scanning optical system, where the beam is scanned at a constant angular speed such as a polygon mirror, a so-called f.theta. lens system having an f.theta. characteristic (a theoretical image height is given by f.theta. where f is a focal distance of the optical system and .theta. is an incident angle) is used as a focusing lens in order to maintain the constant speed in a main scan direction on the image plane.
In general, since the image plane which is scanned by the beam is planar, an image plane distortion on the image plane is compensated in the focusing lens.
Further, in order to prevent the vibration (ununiformity in pitch) of the scan lines on the image plane due to skew of a reflection plane of a deflector such as a polygon mirror from a predetermined position, an anamorphic optical system such as toric lens is sometimes used in the focusing lens.
An f.theta. lens having three lenses in the focusing lens system in order to form a fine laser spot (less than 50 .mu.m by a laser having a wavelength .lambda.=780 nm) has been proposed in U.S. Pat. No. 4,674,825. This f.theta. lens has a construction shown in FIG. 1 in which a concave spherical lens 51, a convex spherical lens 52 and a toric lens 53 are arranged in the order from a mirror plane M of a deflector to an image plane I.
Data of the scanning lens shown in FIG. 1 is shown below.
TABLE 1 ______________________________________ Data of Scanning Lens ______________________________________ R.sub.1 = -31.905 D.sub.1 = 4.70 N.sub.1 = 1.51072 R.sub.2 = -156.190 D.sub.2 = 2.095 N.sub.2 = 1 R.sub.3 = -107.660 D.sub.3 = 16.7 N.sub.3 = 1.76591 R.sub.4 = -52.701 D.sub.4 = 1.0 N.sub.4 = 1 R.sub.5.sup.( *.sup.1) = .infin. D.sub.5 = 16.1 N.sub.5 = 1.78569 R.sub.6.sup.( *.sup.1) = -131.56 f = 170.4 mm image angle .+-.37.5.degree. F.sub.NO = 4 wavelength 780 nm ______________________________________ .sup.( *.sup.1) Toric lens. In a subscan direction, R.sub.5 = -157.46 R.sub.6 = -38.208
In the Table 1, Ri is a radius of curvature of the i-th lens plane as counted from the mirror plane M of the deflector, Di is a plane-to-plane distance from the i-th lens plane to the (i+1)th lens plane, and Ni is a refractive index of a medium behind the i-th lens plane.
However, in the above three-lens f.theta. lens system, a spot shape is materially degraded and the shape may be distorted to a triangle in a peripheral area of a large image angle (scanning angle) image plane even if the f.theta. characteristic, the image plane distortion and the skew correction are met. As a result, an effective spot diameter increases. Thus, in the prior art lens system, a high quality image is not attained in the peripheral area of the image plane.
The image plane distortion of the f.theta. lens shown in FIG. 1 and the f.theta. characteristic are shown in FIGS. 2 and 3. A spot shape on the image plane at an image angle of 37 degrees is shown in FIG. 4.
As seen from those charts, the f.theta. characteristic and the image plane distortion are well compensated in the present example but the spot shapes of 1/e.sup.2 (spot shapes from the peak intensity to 1/e.sup.2 intensity) are triangular and good spot shape is not attained. As a result, in this f.theta. lens, the quality of the image in the peripheral area of the image plane is degraded.